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Ice-sheet models are a powerful tool to project the evolution of the Greenland and Antarctic ice sheets and thus their future contribution to global sea-level changes. Testing the ability of ice-sheet models to reproduce the ongoing and past evolution of the ice cover in Greenland and Antarctica is a fundamental part of every modelling effort. However, benchmarking ice-sheet model results against real-world observations is a non-trivial process as observational data come with spatiotemporal gaps in coverage. Here, we present a new approach to assess the accuracy of ice-sheet models which makes use of the internal layering of the Antarctic ice sheet. We calculate isochrone elevations from simulated Antarctic geometries and velocities via passive Lagrangian tracers, highlighting that a good fit of the model to two-dimensional datasets such as surface velocity and ice thickness does not guarantee a good match against the 3D architecture of the ice sheet and thus correct evolution over time. We show that palaeoclimate forcing schemes derived from ice-core records and climate models commonly used to drive ice-sheet models work well to constrain the 3D structure of ice flow and age in the interior of the East Antarctic ice sheet and especially along ice divides but fail towards the ice-sheet margin. The comparison to isochronal horizons attempted here reveals that simple heuristics of basal drag can lead to an overestimation of the vertical interior ice-sheet flow especially over subglacial basins. Our model observation intercomparison approach opens a new avenue for the improvement and tuning of current ice-sheet models via a more rigid constraint on model parameterisations and climate forcing, which will benefit model-based estimates of future and past ice-sheet changes.

Understanding the temporal variation of cosmic radiation and solar activity during the Holocene is essential for studies of the solar-terrestrial relationship. Cosmic-ray produced radionuclides, such as 10Be and 14C which are stored in polar ice cores and tree rings, offer the unique opportunity to reconstruct the history of cosmic radiation and solar activity over many millennia. Although records from different archives basically agree, they also show some deviations during certain periods. So far most reconstructions were based on only one single radionuclide record, which makes detection and correction of these deviations impossible. Here we combine different 10Be ice core records from Greenland and Antarctica with the global 14C tree ring record using principal component analysis. This approach is only possible due to a new high-resolution 10Be record from Dronning Maud Land obtained within the European Project for Ice Coring in Antarctica in Antarctica. The new cosmic radiation record enables us to derive total solar irradiance, which is then used as a proxy of solar activity to identify the solar imprint in an Asian climate record. Though generally the agreement between solar forcing and Asian climate is good, there are also periods without any coherence, pointing to other forcings like volcanoes and greenhouse gases and their corresponding feedbacks. The newly derived records have the potential to improve our understanding of the solar dynamics and to quantify the solar influence on climate.

During deglaciations, Earth takes up vast amounts of energy, about half of which heats the global ocean. Thus, ocean heat content (OHC) is a key metric to assess Earth's energy budget. Recent modeling studies suggest that OHC changes not only in response to orbitally driven climate change but is also modulated on millennial timescales by the Atlantic Meridional Overturning Circulation (AMOC). Here, we present the first OHC record for the last four deglaciations using noble‐gas ratios in the EPICA Dome C ice core. The record reveals millennial‐scale OHC variability in all studied deglaciations, most prominently as OHC maxima at the end of Terminations II, III, and IV. These millennial‐scale OHC changes are anti‐correlated with AMOC strength, suggesting that the AMOC modulates OHC across different climate states. Furthermore, given the magnitude of the end‐of‐termination OHC maxima, AMOC‐induced OHC changes may be an important control of early interglacial atmospheric CO2, sea level, and climate. Plain Language Summary Earth's energy balance determines whether the planet experiences a net gain or loss of energy. Due to its high heat capacity, the ocean is one of Earth's dominant energy reservoirs and ocean heat content (OHC) therefore a key metric to assess the global energy balance. Recently, OHC has been suggested to not only change on glacial–interglacial timescales, but to also be affected by ocean circulation on millennial timescales. Our OHC record reveals that millennial‐scale OHC changes occur concomitantly with changes in ocean circulation across different climate states during the last four glacial–interglacial transitions. This suggests that ocean circulation plays a crucial role controlling millennial‐scale ocean heat uptake, which has consequences for atmospheric CO2, sea level, and climate. Key Points First ocean heat content (OHC) record covering the last four deglaciations All studied deglaciations show millennial OHC variability anti‐correlated with Atlantic meridional overturning circulation (AMOC) strength Prominent OHC maxima at the ends of Termination II–IV point to AMOC‐induced OHC changes as important control of early interglacial climate

Cover caption The air bubbles trapped in the Antarctic Vostok and EPICA Dome C ice cores provide composite records of levels of atmospheric carbon dioxide and methane covering the past 650,000 years. Now the record of atmospheric carbon dioxide and methane concentrations has been extended by two more complete glacial cycles to 800,000 years ago. The new data are from the lowest 200 metres of the Dome C core. This ice core went down to just a few metres above bedrock at a depth of 3,260 metres. Two papers report analyses of this deep ice, including the lowest carbon dioxide concentration so far measured in an ice core. Atmospheric carbon dioxide is strongly correlated with Antarctic temperature throughout the eight glacial cycles, but with significantly lower concentrations between 650,000 and 750,000 years before present. The cover shows a strip of ice core from an Antarctic ice core from Berkner Island, this slice from a depth of 120 metres. Photo by Chris Gilbert, British Antarctic Survey. Elsewhere in this issue, we move from climates past to future plans for climate prediction. Changes in past atmospheric carbon dioxide concentrations can be determined by measuring the composition of air trapped in ice cores from Antarctica. So far, the Antarctic Vostok and EPICA Dome C ice cores have provided a composite record of atmospheric carbon dioxide levels over the past 650,000 years 1 , 2 , 3 , 4 . Here we present results of the lowest 200 m of the Dome C ice core, extending the record of atmospheric carbon dioxide concentration by two complete glacial cycles to 800,000 yr before present. From previously published data 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 and the present work, we find that atmospheric carbon dioxide is strongly correlated with Antarctic temperature throughout eight glacial cycles but with significantly lower concentrations between 650,000 and 750,000 yr before present. Carbon dioxide levels are below 180 parts per million by volume (p.p.m.v.) for a period of 3,000 yr during Marine Isotope Stage 16, possibly reflecting more pronounced oceanic carbon storage. We report the lowest carbon dioxide concentration measured in an ice core, which extends the pre-industrial range of carbon dioxide concentrations during the late Quaternary by about 10 p.p.m.v. to 172–300 p.p.m.v.

The energy imbalance at the top of the atmosphere determines the temporal evolution of the global climate, and vice versa changes in the climate system can alter the planetary energy fluxes. This interplay is fundamental to our understanding of Earth’s heat budget and the climate system. However, even today, the direct measurement of global radiative fluxes is difficult, such that most assessments are based on changes in the total energy content of the climate system. We apply the same approach to estimate the long-term evolution of Earth’s radiative imbalance in the past. New measurements of noble gas-derived mean ocean temperature from the European Project for Ice Coring in Antarctica Dome C ice core covering the last 40,000 y, combined with recent results from the West Antarctic Ice Sheet Divide ice core and the sea-level record, allow us to quantitatively reconstruct the history of the climate system energy budget. The temporal derivative of this quantity must be equal to the planetary radiative imbalance. During the deglaciation, a positive imbalance of typically +0.2 W·m−2 is maintained for ∼10,000 y, however, with two distinct peaks that reach up to 0.4 W·m−2 during times of substantially reduced Atlantic Meridional Overturning Circulation. We conclude that these peaks are related to net changes in ocean heat uptake, likely due to rapid changes in North Atlantic deep-water formation and their impact on the global radiative balance, while changes in cloud coverage, albeit uncertain, may also factor into the picture.

The ocean's immense ability to store and release heat on centennial to millennial time scales modulates the impacts of climate perturbations. To gain a better understanding of past variations in mean ocean temperature (MOT), a noble gas‐based proxy measured from ancient air in ice cores has been developed. Here we assess non‐temperature effects that may influence the atmospheric noble gas ratios reconstructed from polar ice and how they impact the temperature signal with an intermediate complexity Earth system model. We find that changes in wind speed, sea‐ice extent, and ocean circulation have partially compensating effects on mean‐ocean noble gas saturation, leading to a slight reduction of noble gas undersaturation at the Last Glacial Maximum (LGM). Taking these effects and ice core measurements into account, our model suggests a revised MOT difference between the LGM and pre‐industrial of −2.1 ± 0.7°C that is also in improved agreement with other independent temperature reconstructions. Plain Language Summary Most of the heat added to the climate system by anthropogenic climate change is taken up by the oceans. To better understand how the ocean responds to climate change over hundreds to thousands of years, an indirect measure for the mean ocean temperature (MOT) based on the temperature‐dependent solubility of noble gases has been developed. Noble gas concentrations of the past atmosphere are archived in air bubbles in polar ice cores, which have been used to reconstruct the MOT of the past 20,000 years when Earth's climate was propelled out of the last ice age. However, uncertainties remain regarding critical parameters that are required to derive the correct MOT of the past. Here we make use of an Earth system model that explicitly simulates the noble gases and allows us to assess these parameters in detail under modern and past climate conditions. We find that changes in wind, sea‐ice, and ocean circulation all play important roles in the partitioning of noble gases between the atmosphere and ocean. By taking these effects into account our model suggests a revised best‐estimate MOT cooling of the last ice age to −2.1 ± 0.7°C, which is about 0.5°C warmer than previous estimates. Key Points Revised implementation of noble gases in Bern3D model tuned to observations of saturation anomalies Complex interplay between air‐sea gas exchange, overturning circulation, and noble gas saturation Including saturation effects noble gas‐based mean ocean temperature of the last glacial maximum is 2°C colder than the Holocene

Important elements of natural climate variations during the last ice age are abrupt temperature increases over Greenland and related warming and cooling periods over Antarctica. Records from Antarctic ice cores have shown that the global carbon cycle also plays a role in these changes. The available data shows that atmospheric CO ₂ follows closely temperatures reconstructed from Antarctic ice cores during these variations. Here, we present new high-resolution CO ₂ data from Antarctic ice cores, which cover the period between 115,000 and 38,000 y before present. Our measurements show that also smaller Antarctic warming events have an imprint in CO ₂ concentrations. Moreover, they indicate that during Marine Isotope Stage (MIS) 5, the peak of millennial CO ₂ variations lags the onset of Dansgaard/Oeschger warmings by 250 ± 190 y. During MIS 3, this lag increases significantly to 870 ± 90 y. Considerations of the ocean circulation suggest that the millennial variability associated with the Atlantic Meridional Overturning Circulation (AMOC) undergoes a mode change from MIS 5 to MIS 4 and 3. Ocean carbon inventory estimates imply that during MIS 3 additional carbon is derived from an extended mass of carbon-enriched Antarctic Bottom Water. The absence of such a carbon-enriched water mass in the North Atlantic during MIS 5 can explain the smaller amount of carbon released to the atmosphere after the Antarctic temperature maximum and, hence, the shorter lag. Our new data provides further constraints for transient coupled carbon cycle-climate simulations during the entire last glacial cycle.

Atmospheric methane concentration shows a well-known decrease over the first half of the Holocene following the Northern Hemisphere summer insolation before it started to increase again to preindustrial values. There is a debate about what caused this change in the methane concentration evolution, in particular, whether an early anthropogenic influence or natural emissions led to the reversal of the atmospheric CH4 concentration evolution. Here, we present new methane concentration and stable hydrogen and carbon isotope data measured on ice core samples from both Greenland and Antarctica over the Holocene. With the help of a two-box model and the full suite of CH4 parameters, the new data allow us to quantify the total methane emissions in the Northern Hemisphere and Southern Hemisphere separately as well as their stable isotopic signatures, while interpretation of isotopic records of only one hemisphere may lead to erroneous conclusions. For the first half of the Holocene our results indicate an asynchronous decrease in Northern Hemisphere and Southern Hemisphere CH4 emissions by more than 30 Tg CH4 yr−1 in total, accompanied by a drop in the northern carbon isotopic source signature of about −3 ‰. This cannot be explained by a change in the source mix alone but requires shifts in the isotopic signature of the sources themselves caused by changes in the precursor material for the methane production. In the second half of the Holocene, global CH4 emissions increased by about 30 Tg CH4 yr−1, while preindustrial isotopic emission signatures remained more or less constant. However, our results show that this early increase in methane emissions took place in the Southern Hemisphere, while Northern Hemisphere emissions started to increase only about 2000 years ago. Accordingly, natural emissions in the southern tropics appear to be the main cause of the CH4 increase starting 5000 years before present, not supporting an early anthropogenic influence on the global methane budget by East Asian land use changes.

The last glacial period is characterized by a number of millennial climate events that have been identified in both Greenland and Antarctic ice cores and that are abrupt in Greenland climate records. The mechanisms governing this climate variability remain a puzzle that requires a precise synchronization of ice cores from the two hemispheres to be resolved. Previously, Greenland and Antarctic ice cores have been synchronized primarily via their common records of gas concentrations or isotopes from the trapped air and via cosmogenic isotopes measured on the ice. In this work, we apply ice core volcanic proxies and annual layer counting to identify large volcanic eruptions that have left a signature in both Greenland and Antarctica. Generally, no tephra is associated with those eruptions in the ice cores, so the source of the eruptions cannot be identified. Instead, we identify and match sequences of volcanic eruptions with bipolar distribution of sulfate, i.e. unique patterns of volcanic events separated by the same number of years at the two poles. Using this approach, we pinpoint 82 large bipolar volcanic eruptions throughout the second half of the last glacial period (12–60 ka). This improved ice core synchronization is applied to determine the bipolar phasing of abrupt climate change events at decadal-scale precision. In response to Greenland abrupt climatic transitions, we find a response in the Antarctic water isotope signals (δ18O and deuterium excess) that is both more immediate and more abrupt than that found with previous gas-based interpolar synchronizations, providing additional support for our volcanic framework. On average, the Antarctic bipolar seesaw climate response lags the midpoint of Greenland abrupt δ18O transitions by 122±24 years. The time difference between Antarctic signals in deuterium excess and δ18O, which likewise informs the time needed to propagate the signal as described by the theory of the bipolar seesaw but is less sensitive to synchronization errors, suggests an Antarctic δ18O lag behind Greenland of 152±37 years. These estimates are shorter than the 200 years suggested by earlier gas-based synchronizations. As before, we find variations in the timing and duration between the response at different sites and for different events suggesting an interaction of oceanic and atmospheric teleconnection patterns as well as internal climate variability.

The international ice core community has a target to obtain continuous ice cores stretching back as far as 1.5 Myr. This would provide vital data (including a CO2 profile) allowing us to assess ideas about the cause of the Mid-Pleistocene Transition (MPT). The European Beyond EPICA project and the Australian Million Year Ice Core project each plan to drill such a core in the region known as Little Dome C. Dating the cores will be challenging, and one approach will be to match some of the records obtained with existing marine sediment datasets, informed by similarities in the existing 800 kyr period. Water isotopes in Antarctica have been shown to closely mirror deepwater temperature, estimated from Mg/Ca ratios of benthic foraminifera, in a marine core on the Chatham Rise near to New Zealand. The dust record in ice cores resembles very closely a South Atlantic marine record of iron accumulation rate. By assuming these relationships continue beyond 800 ka, our ice core record could be synchronised to dated marine sediments. This could be supplemented, and allow synchronisation at higher resolution, by the identification of rapid millennial-scale events that are observed both in Antarctic methane records and in emerging records of planktic oxygen isotopes and alkenone sea surface temperature (SST) from the Portuguese Margin. Although published data remain quite sparse, it should also be possible to match 10Be from ice cores to records of geomagnetic palaeo-intensity and authigenic 10Be/9Be in marine sediments. However, there are a number of issues that have to be resolved before the ice core 10Be record can be used. The approach of matching records to a template will be most successful if the new core is in stratigraphic order but should also provide constraints on disordered records if used in combination with absolute radiogenic ages.